The present invention relates to an optical apparatus which enables high-speed measurement of profiles of both sides as well as a thickness variation of a wafer.
A thickness variation of a wafer has been measured so far by either an optical method using interference fringes or a physical method using displacement sensors for scanning both sides of the wafer. The term thickness variation in this specification means a thickness distribution calculated as a distribution in height of one surface from the other surface of the wafer which is regarded as an ideal flat plane.
In the conventional optical method, thickness variation is calculated from interference fringes which occur between a referential plane of an optical lens and a surface of a wafer. The method enables high-speed measurement, but uses a vacuum chuck for holding the wafer. When the wafer is fixed to the vacuum chuck, its backside attracted to the vacuum chuck is likely deformed. Deformation of the wafer at the backside may be incorporated as an error in measurement results, so that the thickness variation of the wafer can not be obtained with high accuracy. There is also a defect that chucking flaws are likely formed on the backside, since the wafer is held in direct contact with the vacuum chuck.
Use of interference fringes derived from light beams reflected on both sides of a wafer is for detecting a thickness variation of a wafer is disclosed in Japanese Patent Application Laid-Open 1-143906. In this method, light beams discharged from a light source are split to transmitting and reflecting beams by a beam splitter, reflected on both sides of a wafer and then inputted to light detectors. Optical interference fringes occur in correspondence with a difference in an optical path between the transmitting and reflecting beams.
On the other hand, in the physical method using displacement sensors, a deviation in thickness of a wafer is calculated on the basis of signals detected by capacitance type displacement sensors provided at both sides of the wafer, to detect a thickness variation of the wafer regarding a back side as an ideal flat plane. For instance, Japanese Patent Publication No. 5-77179 discloses provision of displacement sensors faced to both sides of a wafer for production of deviation signals from every part of the wafer being rotated.
The physical method using displacement sensors has been commonly used so far for detecting a thickness variation of a wafer, since measurement is performed with high reproductivity without any defects caused by a vacuum chuck. However, the wafer necessarily rotated for scanning due to a small probe of the displacement sensor, so that it takes a therefor long time to scan the whole surface of the wafer.
When a part of the wafer attracted to a vacuum chuck is scanned, the wafer is re-held, and then movement of the sensors is changed to a swinging mode for scanning the part which was attracted with the vacuum chuck, as disclosed in Japanese Patent Publication No. 5-77179. The re-holding prolongs a measuring time in total and needs troublesome works.
During measuring, the wafer is rotated. Due to rotation, both sides of the wafer come in contact with a large quantity of the air, so that the wafer is exposed to adhesion of particles suspended in the air. Particles are also transferred from the vacuum chuck to the backside of the wafer. Adhesion of particles often unfavorably affects therefor the measurement results.
When a wafer is scanned with displacement sensors, an outer part of the wafer is not subjected to scanning in order to avoid incorporation of edge effects into detected signals. That is, the outer part of the wafer is treated as an unmeasurable zone, resulting in reduction of a surface part which can be measured.
A wafer for measurement is held by attracting its backside center to a vacuum chuck. Due to this holding means, measurement results are likely affected by gravity as enlargement of the wafer in size. Deformation of the wafer at its periphery is often incorporated as an error into measurement results.
Defects caused by holding a wafer with a vacuum chuck is eliminated by an optical method of measuring a thickness variation of a wafer in such a state kept free from a holding force, as disclosed in Japanese Patent Application Laid-Open 1-143906. According to this method, measurement is performed with ease in a short time, since the thickness variation is calculated from interference fringes. However, the interference fringes which occur between transmitted and reflected light beams reflected on both sides of the wafer are used for calculation of the thickness variation. Consequently, the thickness variation is merely judged from the interference fringes, but undulation or inclination of the wafer which is not accompanied with a thickness deviation can not be detected. In addition, affections of particles floating in the air, positioning of a wafer, dimensional accuracy of various parts to a measuring apparatus, etc. are likely incorporated as errors into measurement results due to a long light path necessarily designed for occurrence of interference fringes.